Iron Ion Sensor Based on Functionalized ZnO

Nanorods

Kimleang Khun, Zafar Hussain Ibupoto, Syed Usman Ali, Chan Ouern Chey,

Omer Nur and Magnus Willander

Linköping University Post Print

N.B.: When citing this work, cite the original article.

This is the authors’ version of the original publication:

Kimleang Khun, Zafar Hussain Ibupoto, Syed Usman Ali, Chan Ouern Chey, Omer Nur and

Magnus Willander, Iron Ion Sensor Based on Functionalized ZnO Nanorods, 2012,

Electroanalysis, (24), 3, 521-528.

http://dx.doi.org/10.1002/elan.201100494

Copyright: Wiley-VCH Verlag Berlin

http://www.wiley-vch.de/publish/en/

Postprint available at: Linköping University Electronic Press

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-76018

1

Iron ion sensor based on functionalised ZnO nanorods

Kimleang Khun1, Zafar Hussain Ibupoto, Syed Muhammad Usman Ali, Chan Oeurn Chey,

Omer Nur, Magnus Willander

Physical Electronics and Nanotechnology Division, Department of Science and Technology,

Campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden

Abstract

In this work, we are presenting an iron ion (Fe3+

) potentiometric sensor based on

functionalized ZnO nanorods with selective ionophore (18 crown 6). Zinc oxide nanorods

with a diameter of about 100 to 150 nm and 1µm in length were grown on gold coated glass.

The selective Fe3+

ionophore sensor with highly aligned ZnO nanorods has given high

sensitivity, acceptable selectivity, reproducibility and stable signal response for detecting

Fe3+

. The potentiometric response of the Fe3+

sensor with functionalized ZnO nanorods

versus a Ag/AgCl reference electrode was observed to be linear over a logarithmic

concentration range from 10-5

M to 10-2

M. The detection limit of the proposed sensor was

about 5µM, which is lower than the normal blood concentration of Fe3+

which is about 10µM

and can be up to 30µM. The sensitivity of proposed Fe3+

sensor was found to be 70.2±2.81

mV/decade with regression coefficient R2 = 0.99 and a response time less than 5 seconds.

The functionalized ZnO nanorods with selective iron ionophore has a life time greater than

one month and has shown insignificant interference with other ions usually present in the

human blood serum. The proposed sensor was used as an indicator electrode for

potentiometric titration.

Key words: Immobilization, Ionophore (18 crown 6), Potentiometric chemical sensors, and

ZnO nanorods.

1 Corresponding author at: Physical Electronics Group, Physical Electronics and Nanotechnology

Division, Department of Science and Technology, Campus Norrköping, Linköping University,

SE-60174 Norrköping, Sweden. Tel.: +46 11363119.

E-mail address: kimleang.khun@liu.se (Khun Kimleang).

2

1. INTRODUCTION

In recent past years, one of the most important semiconductors in research is zinc

oxide (ZnO). This is due to its many one-dimensional nanostructures such as nanorods,

nanotubes, nanowires and nanobelts beside the many interesting properties. These

nanostructures have been highly important for nano-photonic applications such as light

emitting diodes, optical waveguides and nanolaser [1-7]; and to sensor applications like gas

sensors [8, 9] and chemical and biosensors [10, 11]. However, the high surface to volume

ratio of the former is the difference between an epilayer and a nanostructure of the same

material. This high surface to volume ratio is important for sensor applications [8-11].

In addition, understanding the surface chemical inceptions in ZnO nanostructures are

very important for sensor devices, due to the fact that sensor devices depend on the

ownerships of surfaces. As an example, the hydroxyl (OH) radical on the tetrahedral

geometry of ZnO restricts the surface assimilation of ethanol [12, 13]. The high proportion of

OH radical can decreases the surface activity of the ZnO sensor, stops the reaction sites for

ethanol molecules and gives less sensing signal.

Iron is a vital element in the human body and is taking effective role in oxygen

transport, storage and also in electron transport [14, 15]. The enzymes which are taking part

in the synthesis of amino acids, hormones and neurotransmitters need Fe3+

. There is around

10-15 mg of iron to be exhibited in the food daily intake, and studies report that the normal

subjects assimilate around 10% of the amount of iron from the food [16]. Due to the

deficiency of iron, the amount of red blood cells in the body reduces and can become a cause

of anaemia. In addition, the surplus amount of iron is stored in the heart, liver and other

organs [17, 18] and this extra iron cannot be spontaneously released from the body, but it is

stored as mentioned above and can put other organs at risk of impairment [19]. Moreover,

excess or less iron compounds in the human body are also cancer causing factors [20]. It is

therefore very important for clinical, environmental and industrial purposes to efficiently

detect Fe3+

.

There are many methods for the detection of iron ions such as atomic absorption

spectroscopy (AAS) [21], inductively coupled plasma (ICP) [22], etc. But these methods

have many limitations such as high cost and instability if a large number of samples analysis

is needed [21, 22]. Moreover, the potentiometric based sensing method is simple,

inexpensive, rapid and more reliable for the analysing for ions detection. In the literature, it is

also reported that ion selective electrodes (ISEs) was used for the determination of cations

3

[23-28] as well as anions [29-31] and is also used for pharmaceutical compounds [32-36],

with some research also reported on Fe3+

detection [37-43]. It is clear that it is necessary to

improve the sensitivity of Fe3+

sensors and develop selective electrodes having relatively

quick response for the determination of the Fe3+

concentration; especially when small

volumes of the sample is available.

In the present study, a Fe+3

sensor based on functionalised ZnO nanorods with 18

crown-6 is developed and the sensor was highly sensitive and can be used for small volumes

of analyte solutions.

2. Experiment

2.1. Materials

Chemicals 18-crown-6 (18CE6) [Fluka] was used as the ionophore for iron ion

selectivity, while Dioctyl phenylphosphonate (DOPP) [Aldrich] was used as plasticizer.

Polyvinylchloride (PVC) [Fluka] was used as the membrane matrix, ferric chloride

hexahydrate FeCl3·6H2O (sigma Aldrich) and trahydrofuran (THF) [Fluka] were used as

solvents. All chemicals were of analytical grade.

2.2. The Fabrication and Preparation of the ZnO Nanorods and their

Morphological Characterisation

The growth of the ZnO nanorods and fabrication of the substrate is given below. A

glass substrate (70×30 mm2) was first cleaned with deionized water and it was then dried by

air. After that the glass substrate was placed on a flat support inside a vacuum chamber

(Evaporator Satis CR725). We evaporated 30 nm thickness of titanium followed by 120 nm

of gold. Before growth, the gold coated glass was first washed with isopropanol, followed by

deionized water and was again dried in air. ZnO nanorods were grown onto the gold coated

glass by using the aqueous chemical growth method [44]. First a uniform layer of a seed

solution (zinc acetate dehydrate) was spin coated on the gold coated glass at 3000 rpm, and

then the substrate was annealed at 115oC in an oven for 20 minutes. Then the substrates were

suspended in a Teflon holder and put into an aqueous solution of 0.075M zinc nitrate

hexahydrate [Zn(NO3)2·6H2O] and 0.075M hexamethylenetetramine [C6H12N4]. The beaker

was kept into the oven for 5 to 7 hours at 95 0C. After that the ZnO grown substrates were

taken out from the oven and washed with deionized water, dried by air and the nanostructures

were characterised by field emission scanning electron microscopy (FESEM). The FESEM

results show that the grown ZnO nanorods were dense with good alignment and controlled

4

length as shown in figure 1a. The morphological characteristics of the grown nanorods can be

controlled by changing the growth parameters like the concentration of the seed solution,

growth temperature and the pH of the aqueous solution [45].

2.3. Functionalization of the ZnO Nanorods with Selective Iron

Ionophore

The ionophore membrane was prepared by the following composition, 1% ionophore

(18 crown 6), 33% PVC and 66% plasticizer (DOPP) [46]. An amount of 400mg of these

components were dissolved into 5 ml of trahydrofuran (THF) in 25 ml glass bottle. Then the

ZnO nanorods grown on gold coated glass substrate were vertically dipped into the ionophore

solution for 5 minutes and left for drying for 1-2 hours at room temperature. SEM image of

functionalised ZnO nanorods is shown in figure 1b. All the functionalised ZnO nanorods

sensors were kept at 4 oC when not in use. The proposed Fe

3+ sensors were used as working

electrode for the potentiometric measurements in an electrolytic solution of ferric chloride

hexahydrate with a concentration range from 10-6

M to 10-2

M [46]. A Ag/AgCl was acting as

a reference electrode. The output voltage of this experiment for each concentration of ferric

chloride hexahydrate solution is recorded by using pH meter (model 826 Metrohm). The time

response of the developed sensor electrode was measured using Keithley 2400.

3. Result and Discussion

3.1. Response Time of the Functionalised ZnO Nanorods Iron Selective

Electrode

The construction of the cell potential of the developed Fe3+

sensor presented here can

be shown by the diagram bellow:

Au |ZnO | membrane | testing solution || Cl- |AgCl | Ag

The cell voltage is a function of the concentration of the testing electrolyte solution. This

means that the voltage changes because of the change in concentration of iron ion in the

testing solution. During the measurement, we tested the selective Fe+3

sensor into 10-6

M iron

electrolytic solution and we observed that the output response of the Fe+3

sensor was not

stable. Then this Fe+3

sensor was tested in 5×10-6

M and the proposed Fe+3

sensor responded

with stable output response. Further we checked the response of the functionalised ZnO Fe+3

electrode into 10-5

M to 10-2

M, and observed that the Fe+3

sensor showed very stable output

voltage for this concentration range. This is shown in the calibration curve of the logarithm

concentration of Fe3+

versus the output voltage response. We added 2ml of 10-1

M of KNO3

solution in each testing ferric chloride hexahydrate solution in order to adjust the ionic

5

concentration inside the solution. In all our measurements the proposed Fe3+

sensor obeyed

the Nernst’s equation. Which states:

Where is the upper voltage and is the intercept of the curve with the y-axis of the

calibration curve. The response of the iron ion sensor was about 63 mV for 10-4

M solution of

ferric chloride hexahydrate. We investigated the output voltage and found that the response

did not change with the change of the volume of the testing solution. We repeated the

measurement in volume ranging from 5 ml to 20 ml with the same selective electrode into the

10-4

M testing solution; the constant output voltage was around 63 to 64 mV for each volume

of the testing solution. The sensing mechanism of the electrochemical iron ion sensor

followed the equation below:

[Fe3+

(H2O)6]3+

+18CE6 = (Fe3+

-18CE6)3+

+ 6H2O

(Fe3+

18CE6)3+

+ ZnO = ZnO (Fe

3+-18CE6)

3+

The proposed iron ion sensor has shown good linearity for a wide concentration range

from 10-5

M to 10-2

M of iron ions. We found that the sensitivity of the sensor is about 70.2±

2.81 mV/decade with a regression coefficient R2 = 0.99. The response time of the electrode

was less than 5 seconds as shown in figure 2a and 2b, respectively. These two parameters, the

response time and the sensitivity fully describe the high efficiency of this proposed iron ion

selective electrode.

3.2. Reproducibility, Linearity and Stability of the Sensor

We tested the linearity, stability and reproducibility of proposed sensor by using 4

iron ion selective sensor electrodes prepared separately using the same procedure, the relative

standard deviation of the functionalized ZnO nanorods iron sensor in the known

concentration of ferric chloride hexahydrate solution was varying with less than 3%. This

indicated a good reproducibility. The reproducibility from one iron sensor to another iron

sensor in a 10-4

M ferric chloride hexahydrate solution is shown in figure 3a. We also checked

(three times) the reproducibility of the same iron sensor in ferric chloride hexahydrate

solution and the observed results with long stability and excellent linearity as shown in figure

3b. The proposed sensor was used from time to time and kept at 4 oC for more than four

weeks. The sensor maintained its actual working activity up to 90% and almost gave the same

response towards the iron ion. We investigated the morphology of the used ZnO membrane

6

iron sensor using SEM and the result is given in figure 1c. From the SEM images it was

observed that the ZnO nanorods were slightly affected during the measurements. This is due

to the small change in pH of the testing solution. The solubility of ZnO nanorods can vitiate

the performances of the working electrode.

3.3. The Effect of Temperature and Interference on the Response of the

Functionalized ZnO Nanorods Selective Iron Ion Sensor

The temperature effect on the functionalized ZnO nanorods selective iron ion sensor

was investigated between temperatures of 21 0C to 80

0C using 10

-4M of ferric chloride

hexahydrate solution as shown in figure 4a. We observed that the output voltage was

increasing gradually from 21 0C to 50

0C. Above 50

oC the output tends to decrease and at 80

oC there was a drastic reduction in voltage due to the decrease in the strength of bonds

between the ionophore and the ZnO nanorods and as the temperature increases, the ionic

mobility of the iron ions increases which in result make more resistance to the movement of

the iron ions towards the respective functionalized ZnO nanorods selective electrode. It was

reported that using the present ionophore for constructing Fe3+

sensor would give a maximum

response in a pH between 1 to 3 [46], but above this pH the signal become unstable due to the

possible interference of hydrogen ions [H+]. The interference is one of the most important

measuring parameter for ion selective electrode systems. For these measurements we

prepared a testing solution with concentration ranging from 10-6

to 10-2

M of each interfering

substance, and then performed the potentiometric measurements. The measured output

voltage of Fe3+

sensor in presence of the interfering ions with difference concentrations was

plotted against the logarithm of concentration using the mixed solution method as shown

figure 4b and as well as we calculated the selectivity coefficient (

) for each

interfering ion using the separation method [47] as given in table 1. The possible interfering

substances that we chose for our experiments were Na+, Zn

2+, Cu

2+, K

+, Li

+, Mg

2+ and Ca

2+.

Figure 4b and Table 1 show that no significant interference was observed.

3.4. Potentiometric Titration

The analytical application of Fe3+

sensor based on the functional of ZnO nanorods

was tested by the potentiometric titration of Fe3+

with EDTA solution. A 20 ml (10-4

M) of

testing solution was titrated with 10-4

M EDTA solution. With the addition of EDTA solution

into the testing solution, the Fe3+

concentration and the output voltage were decreased

because of the formation of Fe-EDTA complex as shown in figure 5. In figure 5, which

7

exhibits that the end point represent the stochiometric formulation of Fe-EDTA complex and

it also suggests that the almost all of the iron ions are used up in the formation of Fe-EDTA

complex and so after the equivalent point, the potential response was found to almost be

constant. It is therefore suggested that the proposed sensor based on functionalized ZnO

nanorods can successfully be used as an indicator electrode for the determination of Fe3+

by

potentiometric titration.

Table 1: The logarithm of selective coefficients for Fe 3+

sensor with different interfering ions

in 10-4

M

Interference ions

K1+

0.142

Mg2+

0.114

Li1+

0.027

Zn2+

0.222

Na1+

0.563

Ca2+

0.135

Cu2+

0.560

8

Table 2: Comparison of the result of the proposed Fe 3+

sensor based on ZnO nanorods with previous published work

No. Slope

(mV/decade)

Times respond

(s)

Detection

Limit (M)

Linear range

(M)

Life times Reference

1 19.4 ± 0.5 ≈ 15 6.8 ×10-7

1.0×10-6

- 1.0× 10-1

9 weeks [36]

2 28.5 ≈ 15 - 3.5×10-6

- 4.0×10-2

2 months [37]

3 20.0 15 5.0×10-6

6.3 ×10-6

- 1.0×10-1

2 months [38]

4 60.0 25 - 30 - 1.0×10-6

- 1.0×10-2

3 months [39]

5 30.5 – 32 8 - 15 1.0×10-6

- 4×10-8

5.0×10-7

- 1.0×10-2

≈ 3 months [40]

6 19.4 ± 0.5 - 3.6×10-7

1.0×10-6

- 1.0× 10-2

2 months [41]

7 57.0 30 1.0×10-6

1.0×10-4

- 1.0×10-1

2 months [42]

8 20 20 1.3 ×10-6

1.0×10-5

- 1.0×10-1

2 months [45]

9 70.2 ± 2.81 ≈ 5 5.0×10-6

1.0×10-5

– 1.0×10-2

4 weeks this work

9

4. Conclusions

In this work, we developed a simple, highly sensitive and selective iron ion chemical

sensor based on ZnO nanorods coated by a selective ionophore (18 crown 6). The response

of the sensor increases with increasing the concentrations of the iron ion from 10-5

M to

10-2

M with a slope 70.2±2.81 mV/decade. The potential of the presented proposed iron ion

sensor is due to its large slope, with a regression coefficient R2

= 0.99 and the relatively

small response time obtained (less than 5 seconds). These characteristics of the presented

sensor reflect its high sensitivity. Since this sensor is inexpensive, highly selective, and easy

to handle for new users and it can be applied for monitoring of iron concentration in human

blood serum and in detection of iron from lubrication oils as well as for the environmental

analysis it can be of potential in for applications. This iron sensor based on functionalized

ZnO nanorods can also be applied as an indicator electrode for the potentiometric titration.

Acknowledgements

We are thankful to International Science Programme (ISP), Uppsala University,

Sweden and the Royal University of Phnom Penh (RUPP), Cambodia, who financially

supported to this research work.

10

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12

Figure Captions

Figure 1(a-c). A typical SEM image of ZnO nanorods grown on gold coated glass substrate

using the aqueous chemical growth method. The figure exhibit: (a) the ZnO nanorods without

membrane, (b) the ZnO nanorods immobilized with ionophore before the use, and in (c) the

functionalized ZnO nanorods after use for sensing.

Figure 2(a-b). (a) The calibration curve for iron sensor, and (b) time response of the iron

sensor base on ZnO nanorods in 10-4

M ferric chloride hexahydrate solution.

Figure 3(a-b). (a) Sensor to sensor reproducibility of (n = 4), for electrodes in 10-4

M ferric

chloride hexahydrate solution, and in (b) the response of three different experiments using the

same sensor together a Ag/AgCl reference electrode.

Figure 4(a-b). (a)The output voltage versus with temperature, and (b) response of electrode

against Fe3+

and other interference cations.

Figure 5. The Potentiometric titration curve of 10-4

M Fe3+

(20ml) solution versus with EDTA

(10-4

M).

13

Figure 1.

c

a

b

14

Figure 2.

a)

2 3 4 5

0

40

80

120

160

200

240V

olt

age

(mV

)

-log[Fe3+

]

b)

0 20 40 60

28

29

30

31

32

Volt

age

(mV

)

Times (s)

15

Figure 3.

a)

1 2 3 4

30

40

50

60

70V

olt

age

(mV

)

Number of electrode

b)

2 3 4 5

0

80

160

240

experiment 1

experiment 2

experiment 3

Volt

age

(mV

)

-log[Fe3+

]

16

Figure 4.

a)

0 15 30 45 60 75 90

10

20

30

40

50

60

70

Volt

age

(mV

)

Temperature (oC)

b)

2 4 6

0

100

200

Volt

age

(mV

)

Log of concentration

Fe3+

K1+

Mg2+

Li1+

Zn2+

Na1+

Ca2+

Cu2+

17

Figure 5.

-4 0 4 8 12 16 20 24 28

0

10

20

30

40

50

60

70

80V

olt

age

(mV

)

Volume of EDTA (ml)